You should probably start by doing free questing/skilling so people can gain your trust and then you can go on with charging a fee.
You can post here if you want to start a skill training shop:
http://www.sythe.org/skill-training/
You can post here if you want to start a questing service shop:
http://www.sythe.org/questing-torso-services/
You should take a look around before making a shop, just to get a feel of what information you should include. These are two you should take a look at:
http://www.sythe.org/questing-torso...king-tijns-07-eoc-questing-torso-service.html
http://www.sythe.org/skill-training/1292803-ziggilys-skilling-service-legit-skill-training.html
If you trade with anything else like gold/memberships you should ALWAYS use a middleman which you can find here:
http://www.sythe.org/middleman-forums/
Also if you're going to be trading with paypal, be careful of chargebacks as they occur often and only accept paypal from people with vouches or you find trustworthy. ALSO! Another thing with vouches make sure you check to see if they're legit, people can just copy and paste other peoples vouches from another persons thread. If there is anything else you need feel free to PM me.